Heretofore, water-soluble quenching media have been being used widely in the art, as being advantageous in that they cause almost no danger of fire, that they have good cooling characteristics and therefore can be used for quenching even low-grade steel, and that they give little soot which may cause environmental pollution. However, they are problematic in that their cooling rate especially at about 300.degree. C. at which martensite transformation starts is too high, resulting in that the quenched objects often have distortion or cracking. In order to overcome this problem, it has heretofore been attempted to add a water-soluble polymer to the quenching bath containing such a water-soluble quenching medium, thereby controlling the cooling rate in the bath to fall within a suitable range.
However, the polymers in water-soluble quenching media are decomposed under heat or through oxidation during use, whereby the cooling characteristics of the media are varied. Specifically, during use of these media, the vapor blanket stage (in which the vapor as generated through the contact of the medium with the high-temperature metal surfaces of the objects being quenched surrounds entirely the objects with the result that the objects are thereafter cooled only through this vapor wall therearound) is shortened and, in addition, the cooling rate in the boiling stage (in which the quenching liquid is kept in direct contact with the objects being quenched to cause active boiling around the objects, and in which the cooling rate is the highest) is enlarged, thereby resulting in that the objects quenched often have distortion or cracking. If a fresh medium is replenished to the quenching bath, the vapor blanket stage is prolonged so that the cooling characteristics of the medium being used can be recovered, but the cooling rate in the boiling stage could not be reduced to such a degree as expected. Even in this case, after all, the objects as quenched in the bath to which a fresh medium has been replenished have still often distortion or cracking although they may have the same degree of hardness as that of the objects as quenched in a fresh quenching bath. At present, a fresh quenching medium is replenished to the bath being used so as to increase the polymer concentration in the bath, thereby prolonging the vapor blanket stage to such a degree that the quenched objects may have an intended hardness while preventing the quenched objects from cracking. However, even though a fresh quenching medium is replenished to the bath being used, the intended quenching becomes difficult to attain in the bath, while the objects quenched therein still often have cracking to overstep the standard for acceptable objects. If so, the bath thus comprising the deteriorated water-soluble quenching medium must be completely exchanged for a fresh one, which, however, is expensive. Given the situation, it is desired to overcome these problems in the art.